If there is a speaker no longer working, this could affect the resistance causing a higher ohm rating than intended. I would just replace it. If it is an old speaker, sometimes they just go bad. Something could have experienced a short or static electricity could have killed it. It could have just stopped working because it wanted to.
1985 rx7 1.1 98k miles
2002 protege 220k miles
1985 rx7 with a 5.0 swap
2002 Protege 292k miles Girlfriend's
4 more running cars in yard not mentioned